1. Field of the Invention
This invention relates to starch-based compositions having a continuous aqueous phase and a dispersed phase comprising water-immiscible organic materials. The invention also relates to a simple and continuous process for preparing these compositions. The dispersed phase is stable and does not separate on prolonged standing. The stability of these compositions with regard to the separation of water and the organic phases is due to the unique cooking process used for their preparation and is achieved without the benefit of emulsifying or dispersing agents. These compositions have unique properties, making them suitable for use as thickening agents, suspending agents, coating materials, adhesives, and as fat substitutes in food products. Moreover, products prepared according to this invention can be dried and subsequently rehydrated to afford compositions having substantially the same properties as the undried dispersions.
2. Description of the Prior Art
Starch is a high molecular weight polymer composed of repeating 1,4-.alpha.-D-glucopyranosyl units (anhydroglucose units, or AGU) and is typically a mixture of linear and branched components. The linear component, amylose, has a molecular weight of several hundred thousand; while the molecular weight of the branched amylopectin is on the order of several million. Although normal cornstarch contains about 25% amylose, cornstarch varieties are available commercially that range in amylose content from 0% (waxy cornstarch) to about 70% (high-amylose cornstarch).
Starch occurs in living plants in-the form of discrete granules ranging from about 5 to 40 microns in diameter, depending on the plant source. It is well known that starch, as isolated from the plant in its native state, is insoluble in water at room temperature because of strong hydrogen bonding between polysaccharide macromolecules. Areas of crystallinity within starch granules also inhibit water solubility. When a water suspension of granular starch is heated, granules at first slowly and reversibly take up water with limited swelling. Then, at a definite temperature, which is typically about 70.degree. C., granules swell rapidly and irreversibly; and areas of crystallinity within the granule are lost. The temperature at which this occurs is commonly referred to as the gelatinization temperature.
Near the gelatinization temperature, a measurable percentage of the starch, in particular the amylose component, becomes soluble and diffuses out of the granule matrix and into the surrounding water. As the temperature is increased beyond about 70.degree. C., a greater percentage of the starch becomes soluble, and the granules become highly swollen and partially disrupted, until, at a temperature of about 90.degree.-100.degree. C., a viscous dispersion of starch in water is obtained. However, despite this outward appearance of solubility, starch is only partially water soluble and exists largely as highly swollen granules and granule fragments that are easily separable from starch solution, for example, by centrifugation. In fact, when cornstarch is heated in water at 95.degree.0 C., only about 25% of the starch actually dissolves the remainder being present as swollen granules and granule fragments.
True solutions of starch in water, with no remaining granules and granule fragments are difficult to prepare using conventional cooking techniques, but can be readily prepared by passing starch-water slurries through a continuous steam jet cooker. Jet cooking has been used commercially for decades to prepare starch solutions for non-food applications, for example, in the paper industry. The method involves pumping an aqueous starch slurry through an orifice where it contacts a jet of high pressure steam. Unlike conventional cooking, which tends to preferentially solubilize the amylose component, steam jet cooking dissolves amylopectin as well as amylose. Somewhat higher starch concentrations than those desired in the final dispersion are used to allow for dilution of the cooked dispersions with condensed steam. Jet cooked starch solutions that have been cooled and dried are often difficult to redisperse in water and generally do not yield lump-free pastes having the smooth consistency required for many applications.
There are basically two types of steam jet cookers used commercially, and these are discussed in an article by R. E. Klein and D. A. Brogly, Pulp & Paper, Vol. 55, p. 98-103, May 1981. The first of these provides a process that is referred to as thermal jet cooking. In this process, the amount of steam added to the aqueous starch slurry is carefully controlled to achieve complete steam condensation during the cooking process. No excess steam is used. The second of these provides a process that is referred to as excess steam jet cooking. In excess steam jet cooking, the steam entering the heating zone of the cooker exceeds the amount required to reach the desired cooking temperature. The turbulence caused by the passage of this excess steam through the heating zone thus acts to promote mechanical shearing of the starch and rupture of polysaccharide molecules, especially those having the highest molecular weight. This not only leads to total and complete polysaccharide solubility but also to a lower apparent viscosity of starch, as compared with either thermal jet cooking or conventional batch cooking.
An inherent property of starch pastes obtained by standard cooking procedures is their tendency to form firm, rigid gels on prolonged standing. The tendency of starch pastes to gel increases with the amylose: amylopectin ratio in the granule. It is generally accepted that gel formation, i.e., retrogradation, is caused by aggregation of starch molecules through hydrogen bonding. Retrogradation and aggregation occurs more readily with amylose than with amylopectin, because amylose is a straight chain polymer with little or no branching. However, under refrigeration, amylopectin will also aggregate over time and will contribute to the gel forming property of starch.
Aqueous dispersions or emulsions of lipid or oil are commonly of the water in oil variety, i.e., lipid or oil is the continuous phase, while the aqueous phase consists of microscopic aqueous droplets uniformly dispersed in the oil. These compositions are normally prepared for use as edible spreads and cannot be dried and subsequently rehydrated. The general method used to prepare these compositions is to first prepare a dispersion of oil in an aqueous phase and to then cause a phase inversion to occur by vigorous mixing. Numerous techniques for the preparation of these water in oil emulsions have been taught in the prior art, for example, in U.S. Pat. Nos.: 4,536,408; 4,849,243; 4,882,187; 4,883,681; 4,917,915; and 5,194,285. Compositions of this general type, where oil is the continuous phase and water is the dispersed phase, are considered to be outside of the scope of this invention.
Starch monoesters of hydrophobic substituted succinates have been used to stabilize emulsions of oil and water, as taught by P. C. Trubiano, in "Modified Starches: Properties and Uses," CRC Press, Boca Raton, Fla., 1986, p. 131; and by O. B. Wurzburg, in Cereal Foods World, Vol. 31, 1986, p. 897. These starch derivatives are prepared by esterifying the hydroxyl substituents of starch through reaction with alkyl, alkenyl, aralkyl or aralkenyl succinic anhydrides. High-solids dispersions of these low-viscosity starch derivatives can be spray-dried to produce a powder containing up to 40-50% encapsulated oil. It has been generally recognized that underivatized starches do not function as oil-water emulsifiers. In this regard, Trubiano, supra, comments on page 139 that in salad dressing, "emulsions made with untreated starch, but having the same viscosity, show very large oil droplets, which may coalesce and separate with time."
Kimball et al. (U.S. Pat. No. 2,471,434) describe the preparation of a dried powdered shortening by mixing flour or starch with water, gelatinizing the starch by heating the mixture to boiling, mixing in an edible fat, and finally spray drying the mixture to yield fat globules enclosed within an encapsulating layer of dried gelatinized flour or starch.
In U.S. Pat. No. 3,769,038, Mitchell et al. describe the preparation of a fat sponge, i.e., a fat containing starch compound that can contain up to 92% fat in an outwardly dry form. Compositions of this type are prepared by adding pregelatinized starch and fat to water, blending to form a dispersion of starch, water, and fat, and then freeze-drying the product. It is significant that the patent teaches that it is preferable not to co-cook starch-fat mixtures but to gelatinize the starch in water first before blending with the fat. Moreover, the fat is not totally emulsified within the starch matrix in the form of fine droplets but may be squeezed out of the sponge by mechanical means.
Bracco (U.S. Pat. No. 4,088,792) describes a process for the preparation of an edible cream, in which an aqueous mixture comprising 10-35% by weight of starch, at least 5% by weight of proteins and at least 5% by weight of fat (of which at least 1% consists of emulsifying fat) is homogenized at a temperature of 70.degree.-90.degree. C., and preferably not exceeding 120.degree. C. The presence of proteins and emulsifying fats, such as monoglycerides and lecithins, are essential, suggesting that without the beneficial emulsifying effects of these substituents, starch alone would not be able to maintain the fat constituent in an emulsified state.
In U.S. Pat. No. 4,159,982, Hermansson describes modified starch products prepared by binding starch with proteins, specifically a casein or caseinate, to form complexes. Complexes are prepared by heating starch with an aqueous dispersion of protein at a temperature exceeding the starch gelatinization temperature. The resulting modified starch does not have the sticky, gummy properties of unmodified starch and also functions as an emulsion stabilizer. The presence of protein in the composition is essential for the emulsion stabilization of lipids, suggesting that starch alone would not function in this capacity.
The preparation of low-fat containing oil-in-water emulsions having the properties of a non-flowable margarine has been described by Miller et al. in U.S. Pat. No. 4,238,520. These compositions preferably contain about 20-28% fat, based on the entire weight of emulsion. Unlike most margarines described in the prior art, fat is the discontinuous phase. In addition to fat, critical components of these compositions are: 1) an oil-soluble or oil-dispersible lipoidal emulsifier, and 2) a water-soluble or water-dispersible thickening agent, such as a starch, gum, or cellulose derivative. Emulsions are prepared by homogenizing the components in water at elevated temperatures followed by cooling.
Bosco et al. (U.S. Pat. No. 4,468,408) describe a stable butter flavored oil-in-water emulsion, useful as a low-fat liquid spread. The composition of Bosco et al. comprises a dispersed phase, containing less than 40% fat, based on the weight of the spread, and a continuous aqueous phase containing 0.1-4% of an emulsifier system comprised of both lipophilic and hydrophilic emulsifiers. The components of the composition are mixed in water, homogenized and cooled to yield the final composition.
In U.S. Pat. No. 4,615,892, Morehouse et al. describe dried compositions prepared from oil-in-water emulsions. These compositions can be reconstituted with water by the consumer to yield a butter-like spread. Oil-in-water emulsions are first prepared by utilizing a low dextrose equivalent (D.E.) starch hydrolysate (D. E. less than 25 and preferably about 5-10) to replace a substantial portion of the oil or fat. The starch hydrolysates (maltodextrins) used to promote formation of the required water-in-oil emulsion are highly soluble in water and exhibit a low tendency to form rigid gels. The emulsions are agitated under conditions selected to prevent phase inversion. Upon drying, the maltodextrin provides a protective film for the fat droplets.
Reimer (U.S. Pat. No. 5,080,921) teaches a low-calorie fat substitute comprising a dispersed phase in the form of protein-lipid aggregates and a continuous aqueous phase containing non-aggregated protein, carbohydrate, and emulsifier. The composition is prepared by mixing the components of the formulation together in water and then applying heat to partially denature the protein.
Fung (U.S. Pat. No. 5,082,684) describes a low-calorie fat substitute prepared by combining an oil or fat with an aqueous phase which is rendered non-flowable by addition of a gel-forming composition, such as a natural gum. Water-binding compositions such as soluble carbohydrates may also be added. Unlike the compositions of the invention described herein, an emulsifier is an essential ingredient of these prior art compositions. In U.S. Pat. No. 5,158,798, Fung et al. teach a carbohydrate fat extender that is added to the composition of Fung, supra; and a portion of the fat is replaced with an incompletely digestible fat mimetic.
Rubens (U.S. Pat. No. 5,149,799) describes an apparatus and method for preparing a spray-dried, pregelatinized starch, in which the resulting starch contains a greater degree of whole, unbroken granules than a starch prepared by conventional spray-drying or drum drying processes. Although it is mentioned that other ingredients such as emulsifiers, flavors, colors, or fats may be added to the starch slurry prior to drying, presumably in minor amounts, the patent teaches that total disruption and solubility of starch granules, such as would be encountered during steam jet cooking, yields inferior products for food applications.
Mallee et al. (U.S. Pat. No. 5,547,513) disclose the preparation of a starch-based texturizing agent from high amylose starch by heat-solubilizing starch granules and filtering to remove impurities such as proteins, fats and other compounds. The resulting texturizing agents are useful in foods as well as in drug and cosmetic formulations.
Doane et al. (U.S. Pat. No. 4,911,952) describe a method for encapsulating various agents, such as agricultural chemicals and food constituents within a starch matrix. After the starch is jet cooked, any of several additives, including vegetable oils, may be blended into the cooked dispersion by slow mixing, such as in a sigma blade mixer.
In U.S. Pat. Nos. 5,131,953 (same as European Patent Application EP 366,898) and 5,435,851, Kasica et al. teach the preparation of pregelatinized starches by a process utilizing thermal jet cooking in a coupled jet cooking/spray drying process. This process produces a pregelatinized starch having a molecular weight, as determined by intrinsic viscosity, that is not substantially less than that of native, uncooked starch. The split second drying time, coupled with the fact that the superheated starch solution is not vented to the atmosphere or cooled prior to spray-drying, minimizes the association of starch molecules (i.e., retrogradation) and yields a starch that readily dissolves or disperses in water. When the starch contains substantial quantities of amylose (e.g., 70% amylose), aqueous dispersions of the pregelatinized starch form firm gels on standing. These gels exhibit gel strengths higher than those observed when drum drying is used, suggesting that drum drying should be avoided if pregelatinized starches with maximum water solubility or dispersibility are sought.
Eden et al. (U.S. Pat. No. 5,236,977) describe a method for preparing a corrugating adhesive composition from a high amylose starch using the coupled jet cooking/spray drying process of Kasica et al., supra. The adhesive optionally contains alkali, borax, crosslinking agents, and other components traditionally used in adhesives.
Ware et al. (U.S. Pat. No. 3,775,144) show a corrugated paperboard adhesive comprising a cooked flour paste of both the starch and protein fractions of the flour, wherein the protein fraction is nondegraded and the starch is essentially chemically unmodified or molecular weight reduced. The flour paste can be produced by cooking a slurry of the flour with steam under the conditions of a high degree of agitation and shear.
Eden et al. (U.S. Pat. No. 4,755,397) teach encapsulation of a variety of materials by combining the material with a high temperature-stabilized pressurized dispersion of starch and precipitating the starch with a salt solution. The process is carried out by combining a slurry of the starch and salt with the material to be encapsulated and cooking the mixture, such as in a jet cooker. Exemplary materials to be encapsulated by this method include flavoring oils, pigments, agricultural agents, bioaffecting compounds, thickeners, and the like.